I have over the years led several overseas camping bike tours with Explorer Scouts (late teens). Therefore I have experience of all sorts of lower end of the range panniers and saddle bags which have had two weeks of heavy teenage abuse thrown at them.
The issue of the backing plate is important, the cardboard and similar one deteriorate very fast and so make the pannier useless very fast, the solution has been to cut up cardboard boxes to make new plates.
The attachment to the rack is critical, I have yet to finish a tour without at least one bike where the pannier is held to the bike by cable ties
I'm a great fan of Obtelieb panniers, one the very rare occasions that something has gone wrong it has been repairable in the field and spare parts are held by most decent bike shops or from the manufacturers web site
The things I don't like about Ortelieb is that they don't have several external pockets as standard.
There are no internal pockets as standard (money/keys etc)
No place to attach lights, wet clothing, maps, food and other stuff the long distance tourer needs
They also look a bit 'odd' in that they don't look like a conventional bag/rucksack.
What I do like about Ortelieb is they are waterproof (really waterproof)
They are tough (almost teenage proof)
They are simple (one hand, even in the dark) to get on and off the bike
They are simple to open (one hand, one clip, can even be opened whilst one the bike)
They are fairly simple to close (two hands though, and you cant do it whist on the bike)
If I were designing a pannier I would start with the Rixen&Kraul or Ortelieb attachment system.
I would have a single main compartment
I would have lots of external webbing like a military rucksack
I would than have a range of pockets and other attachments
I would make it waterproof
I would also build in double thickness 'crash zone' as everyone drops a loaded bike on tour, I have seen a number of panniers damaged in the lower side area